.beta.-Bromocitraconic anhydride has been made heretofore by the dehydrohalogenation and dehydration of .alpha.,.beta.-dibromomethylsuccinic acid (Vaughn and Milton, J. Am. Chem. Soc., 73, 5497-8 (1951)) or by the dehydrohalogenation of .alpha.,.beta.-dibromomethylsuccinic anhydride (Laurser et al., J. Med. Chem., 14, 619-21 (1971)). Vaughn and Milton point out the great ease with which citraconic anhydride isomerizes to itaconic anhydride. In the very labile acid system Itaconic .revreaction. citraconic .revreaction. mesaconic only about 15% of the equilibrium mixture is in the citraconic configuration (Whitmore, Organic Chemistry, D. Van Nostrand, 1937, page 472).